Rain storms in early spring can abruptly break up the ice cover on northern rivers and start it moving rapidly downstream. The broken ice pieces will often lodge against obstructions or stronger ice sheets to form thick ice jams. Ice jams can restrict flow of a river such that water levels can rise 10 feet or more in an hour, often flooding adjacent communities in or near floodplain regions.
Very few methods exist to control breakup ice jams. Large dams with ice retaining piers or tall, closely spaced vertical piers have been proven to arrest a breakup ice run. However, these structures are expensive because they must resist the full load generated by a breakup ice run. In most cases, the cost of such structures exceeds the benefits derived from reduced flooding. They also have the limitations of: i) being extremely intrusive; ii) altering fish habitats by interfering with the natural river flow; and iii) preventing small boat passage, e.g. canoe or small fishing boats. Thus, their use is often unacceptable to other users of the river.
A paper entitled "Ice Control Structures on Slovak Rivers" by Brachtl in Proceedings of International Symposium on River and Ice, International Association of Hydraulic Research, Budapest Hungary, 1974, pp. 149-153, teaches of closely spaced inclined piles to stop ice motion caused by hydroplant peaking. These inclined piles prevent thicker ice accumulations further downstream and hence reduce ice-jam flooding caused by hydroplant operation. These inclined piles are particularly useful on regulated rivers that have dam structures. However, ice motion induced by hydroplant peaking is much less severe than that resulting during breakup of an unregulated river where dam structures are not present. Although this teaching provides general explanation of such piling structures, it does not provide a method of using such structures on unregulated rivers or of a durable structural design for withstanding powerful breakup ice runs characteristic of an unregulated river as required by the instant invention. Moreover, this teaching does not teach or suggest of:i) a need for protection of the foundations of an ice-resisting piling structure from scour during breakup ice runs; ii) whether their piling structures can adequately initiate formation of an induced grounded ice jam, an essential feature for reducing loads on an ice-resisting structure as required by the instant invention; and iii) a wide enough spacing between the ice-resisting piling structures which allows passage of small boats as well as river debris during spring flooding. This latter aspect can be the cause of increased spring flood levels upstream from the piles.
Earlier civil structures for control of ice flows include U.S. Pat. 3,798,912 by Best et al. entitled "Artificial Islands and Method of Controlling Ice Movement in Natural or Man-Made Bodies of Water." This patent teaches of man-made structures for placement along river channels for controlling ice flows at harbor entrances. U.S. Pat. 3,881,318 by Galloway entitled "Arctic Barrier Formation" teaches of a method for placement of civil structures in arctic regions for protection of offshore work platforms from ice flows. Neither of U.S. Patents teach or suggest the instant invention's flood prevention technique for use of multiple sloped-face elements in small rivers located near a surrounding floodplain region for protection of a nearby town.
Thus, the instant invention below provides a method for creating a retained and stabilized ice jam while allowing water bypass at locations upstreem of a town otherwise prone to flooding.